Green Geology

Useful links:

 

Pentire Peninsula

 

South West Coast Path

 

How to get there:

Bus

 

Nearest railway station:

Roche (11 miles)

 

Accessibility:

Long-ish walk along coastal path. 

Devonian and Quaternary

 

Site of Special Scientific Interest (SSSI)

Trebetherick Point, Cornwall

Text and photographs:  Naomi Stevenson

 

Page updated March 2013

Trebetherick Point is a coastal site, ideal for those who like their geology “crunchy” – i.e. recent.  A sequence of Quaternary sediments on a shore platform of Devonian slate is well worth visiting.  The village of Trebetherick (much loved by the Poet Laureate John Betjeman) is near Polzeath between Hayle Bay to the north and Daymer Bay to the south; the geology here is reached via the South west Coastal Path.  A visit to the Point can be combined with a visit to the Pentire Peninsula as a full day’s outing.

 

Walking from Polzeath down to Daymer Bay, the shore platform consists of Polzeath slate to the north of Polzeath Point, and Harbour Cove slate from there southwards.  The Polzeath slate is more solidly purple in colour than the grey/green/purple Harbour Cove formation.  These slates were deposited in a deep-water, low-energy environment.  Unusually for slates, both formations are quite fossiliferous, enabling their deposition to be dated within different parts of the Devonian.  

 

The Pleistocene deposits which overlie the slate have been used to help “unravel” the geological history of south-west England.

 

Immediately above the slate are raised beach sediments – in this instance consisting of sand, gravel, and boulders, with small fragments of slate and larger boulders of greenstone.  The sediments contain many fossil mussel and limpet shells and are interpreted as having been deposited during an interglacial.  

 

The layer above this is a sandrock with alternate harder and softer beds which have resulted from uneven cementation.  The difference in hardness is such that the bedding is very easily distinguished.  This rock is believed to have been deposited as the climate was improving after a relatively cold period.

 

Overlying this, in turn, is a layer of head with angular slate fragments, laid down in glacial conditions.  In places, this layer is directly overlying the shore platform.  

 

At the top of the sequence is a layer of pebbly clay, less than 1m thick, covered by blown Holocene sand which is occasionally cemented.  The evidence is that the clay layer was a wash deposit; its surface represents a former land surface.  The sand contains mussel shells and land snails and flint flakes.  The presence of the snail shells indicates that it was laid down under temperate conditions.

 

One very interesting feature of the sediments at Trebetherick is a very localised boulder gravel – there is much controversy about its origins.   It contains well-rounded pebbles, cobbles and boulders of many different rocks including quartz, basalt, granite and sandstone.  The layer shows no evidence of bedding.  

 

There is strong evidence that the boulder gravel may have glacial origins and that a tongue of ice may have reached the Camel estuary; in this scenario the ice gathered the pebbles and left a patch of till, the eroded gravel being the remnant of this.  It may be that a sheet of ice centred on Lundy was responsible.

 

There is an alternative hypothesis in which the boulder gravel and the pebbly clay layer are both part of the same river gravel (or raised beach) deposit during the Ipswichian interglacial.

 

If this is ever resolved it will make an important contribution to the debate about glaciations in south-west England.

Click to enlarge

Looking down to the foreshore slates